The shop did not decide on the over balance I did. But I won't get into an argument I don't do that very well...I know from 30 years of machine shop experience and fabrication work that physics come into play, I understand that if you balance your teeter totter you would want 50% but if one end of the teeter totter had a cup mounted on it and it started raining 50% won't work very well. Then if one end has a fan blowing at it but sometimes it suck instead of blows it might make a difference. Not to mention that one end is upder a tree and bird are dropping crap on that end, might make a difference over time. I know this is over simplified but people can get goo goo eyed when somebody starts using words and terms they cant understand and they might form an unjustified opinion. Reminds me of an old saying "if you can't dazzle em with brilliance, baffle em with B.S." my point is it is just an opinion and everybody has one. LOL and I can prove that.... Just look at our country today.... I do know that a given amount of weight in an unbalanced situation will increase as rpm increases. ( spin an unbalanced crank by hand, then spin that sucker at 5000 rpms and see what happens) so to say that balance based on math is the only way to go is like wearing blinders. There are way too many factors that we can't prove on paper but if over simplified they make sense. As far as over balance vs under balance goes, it makes sense to me that as rpm increases you would want to be on the minus side ( or have the crank balanced for a heavier rod piston combo) than the other way around. Your error won't get lighter as rpm goes up it will get heavier, more load on the pistons, more oil etc... I am not looking to start a forum pissing match like some want to do but there are some people out there that think they know it all and then there are those of us that don't know anything but just take in as much info as we can get, dig through it all, toss the obvious crap out and use the stuff that that has some logic to back it up not just numbers. Some of the best discoveries were by accident and Some people said that can't be, the numbers don't support it. we all know the earth is round not flat. The following is from Eaton Engine Balancing
If a carbon build-up on the piston top was anticipated over the long haul, then this could be also added to the oil value at this point. If you have a preference for a different oil value to be used on your rotating assembly upon getting it balanced, then talk this over with your shop and get their input on this. Most shops will be agreeable to sutle changes in the bobweight values if you have specific preferences.
There are a variety of other conditions which would require “overbalancing” as part of the balancing process. A change in rod lengths or crankshaft stroke can benefit from a given amount of overbalance depending upon the amount of change in rod/stroke ratio. The use of nitrous oxide, superchargers, or turbo chargers typically also requires a certain amount of overbalance. Using nitro methane in conjunction with a blower is likely the worse case scenario as cylinder pressures are extremely high under detonation which artificially increases the piston weight by a more than a normal amount. Any form of blown engine will benefit from a given amount of overbalance simply due to the weight of the piston averaging artificially heavier not only from the increase in cylinder pressure at ignition, but the increase in cylinder pressure taking place while the cylinder is also filling during the intake stroke. In this instance, the piston is averaging an overall heavier weight when running at speed. A normally aspirated engine has a given amount of pressure counterbalance in that the piston is subjected to negative pressure when the cylinder is filling but is under increased pressure during compression and ignition. If an aspirated engine is working with an extremely well designed induction system and is benefiting from a ramming effect to fill the cylinders at the upper rpm ranges, then overbalancing also helps here. And then there’s the rpm factor. Balancing is linear up to a point throughout the rpm range but depending upon the masses at work within your particular assembly, there is a point in which the crankshaft rpm starts to out run the dynamics of the existing state of balance. Overbalance allows these dynamics to stay in tune or “caught up” to the rpm’s of the crankshaft. There are proprietary formulas that calculate these amounts of overbalance for all the different variables and will vary somewhat from shop to shop. Again, talk with your balance shop regarding overbalancing and determine if this would be best applied to your application.