I think 50 is about right however, please don't use 'flexisyns'. The point of flexisyns is to PLAN AHEAD, a night out for a SPECIAL OCCASSION, with a SPECIFIC LIMIT of how many syns you are going to have. COUNT DOWN from that number and STOP when you have run out. Its not for 'I drank a bit too much on Saturday, Oh never mind, I'll count it as a flexi syn day'. You were off plan. Simple. Wake up the next day, start again. If you really want to, and it won't make you feel deprived, you could cut your syns down to 5 a day for the rest of the week.
Apologies for capitals but I have seriously lost count of the number of times that some suggests flex synning in this type of situation so there is obviously alot of confusion about what it actually means.