I must say that this has been a hot topic at our school in the past year. We ended up not giving money to the principal's fund...instead, we purchase things for the principal so that we have a itemized receipt. That way, we can show exactly what our money has gone towards should anyone (like parents) ask.
We DON"T give the principals any money. They have their own fund raisers for their funds. We do however, include them as PTO Officers and they get a say in how we spend our funds.
A fixed dollar amount does seem strange; who's to say you'll have that money available?
We do have a discretionary line for the principal for use during the year, and at the end of the year, the principal presents a staff wish list for any surplus funds.
We never had a percentage listed but we gave our principal blocks of money at times somewhat for his discretion. (He'd let us know about a list of upcoming expenses/activities.)
I actually can see where the percentage might be a fair idea. Now days it's not just the PTOs that do fundraising, schools usually have them as well. So looking at the big picture, do the school and PTO have separate fundraisers, perhaps even competing ones? The principal/officials get hammered by parents about "too many fundraisers". No matter that that PTO funds are FOR the the school and students, school resources are used to make them a success.
If I were a principal, I'd probably see PTO fundraising as a partnership between the school and the PTO, with some funds allocated for the Principal's discretion.
What amount is fair, if it's a percentage or flat amount, or if it should happen at all are valid points for discussion.
One thing - if that's they way it's been for some while, those funds are fully part of the Principal's budget now. If they go away, he/she will have to either cut expenses or find another way to fund the items.
We do have a budget line for school improvements, this is used at the end of the year by the principal. However, she brings a proposal to us and we as a board vote on it and produce a check to the principal. We get the recognition for ways we are putting parents money to use in our school.
Our elementary school PTO does not allocate money to be used at principle's discretion, but our high school PA does.
At the minimum, I would change the by-laws to reflect a percentage amount rather than an actual dollar amount in the event that fundraising for any given year takes a hit because of the economy or other factors.
However, I don't care for any discretionary funds of a substantial amount put into a parent organization's budget. If the principle (or any entity) has activities or improvements he/she want the organization to fund, he/she ought to plan ahead to bring a specific proposal to the budget setting meeting and seek a motion/vote.