We were always told to have flexi syn day only when you need it. For example - a work party out where you know you'll want to eat fatty food and drink alcohol.
I don't think you should give yourself an "allowence" of flexi syn days, I think the main point of them is letting yourself have a "day off" and making sure you get back on it the next day.
Otherwise, you may give up and think you've runied it all (by having lots of syns) and then it's on to the slippery slope of missing WI's etc. xx