Hi foxwood.
As nobody's replied i will give it a go.
To me backfitting a system is changing its original rules to include winners that weren't originally their and eliminating losers that were during a survey.
Back tweaking as you call it is retaining the original rules of a system but adding a filter within that survey period to improve ones profitability.
Be careful now as with your new rule it's now a new system.
You now need to do a forward survey say starting from when the original rule was introduced to see if the improved s/rate + profit levels stay the same.
If they do allow for minor variances either way, then you have the nucleas of a good system.
All that's required now is what you would consider to be a reasonable amount of bets for it to sustain long term profits.
cheers
garyf.