The % price limit doesn't work consistently, and so can't be used.
What I'm suggesting is a little different than that, though.
Say I have $100K cash. I only want to take positions with a max size of $15K. I'd like to be able to configure this, so the system would automatically reject orders larger than $15K, just as it does when you attempt to exceed margin limits. In other words, for the purpose of new orders, instead of allowing them to be up to $400K, allow them to only be up to $15K.
Better still, instead of outright rejecting the order, a nice feature would be to trim the size (in 100s if starting with a round lot order, in 1s if starting with an odd-lot) so the order is under the limit.