Ah, thanks for the tip about the "help" keyword, I only knew what 3DFibs gave me. I'd thought that the formula was more complex than just the sum of experiences, ie. that it took the reputation of the voucher/complainer into account too.
The formula info didn't mention it being dynamic so I tested it with someone who I vouched for a while back. I asked for their rep, got 12356, withdrew and asked, got 11940, then revouched and asked and ended with 12583. That resulted in a boost of 227 so it looks like you can
top someone up every now and then!