Here's my concern:
if he decides to convert the loan, that would dilute the two other
partners and he would get 72.5% of the shares.
Is that correct?
This depends upon exactly how the conversion right is worded. It would be unusual, but not unprecedented, for a conversion right to depend upon the current value of the company. More often, the conversion right would establish a fixed number of shares for which $X of debt could be converted if Y and Z conditions are met.
and has anyone seen 64% as a 'common' threshold?
A 64% threshold is very unusual.
Under Robert's Rules of Order, there are many things that can be done only with a two-third's threshold. But, those rules contemplate a situation where there are many equal participants in voting.
The percentage threshold for extraordinary actions in a closely held company is, instead, usually individually negotiated on a case by case basis, usually so that:
(1) more individual shareholders (or a the same number of shareholders with a larger percentage interest), in practice and as contemplated, have to vote yes than are required to secured an ordinary majority vote (otherwise it really isn't a supermajority at all), and
(2) so that either none of the major shareholders or no more than one of the major shareholders can dissent for a matter to pass, but so that minor shareholders objections can be overruled.
The exact percentage varies based upon the circumstances and there is no one "common" percentage across the board.
In a situation like this one, assuming that exercising the conversion right would give him 72.5% of the shares (leaving the remaining shareholders with a 22.5% and a 5% share respectively after dilution), any percentage above 72.5% and not in excess of 77.5% would require at least two votes for supermajority action post-conversion, any percentage in excess of 77.5% would require the converter and the now 22.5% shareholder to approve, and any supermajority in excess of 95% would require a unanimous vote post-conversion.
Pre-conversion, a majority requirement would require any two shareholders to approve, any percentage in excess of 55% and not in excess of 90% would require both 45% shareholders to approve, and any percentage in excess of 90% would require a unanimous pre-conversion vote.
Normally, in a case like that, the requirement for a supermajority would usually be any percentage in excess of 77.5% and not in excess of 90%, and a supermajority would almost never be 72.5% or less. A cut off of 80% or 85% would be pretty common in that situation (to leave room for overriding some future new small percentage interest shareholder like a key employee as well), a cut off of 75% would be uncommon but not unheard of, and a 64% supermajority would be very unusual in a case like that unless the debt converter has great economic power in the relationship (e.g. the deal just can't go forward without him).
