Roughly speaking theres about 26 syns in a bottle, so 39 in a bottle and a half. Take away 15 for today, thats 24 over. Cut back to 5 syns a day over the weekend and you make up 20 of those. You can then have 11 on Monday and the balance is restored.
Much as I sound like a stuck record, flexisynning is a pre-planned method of staying in control of what you consume. This isnt it. Writing something off as a flexisyn after the event does not reinforce the message that you are in control of your consumption.
But in all honesty, I wouldnt worry about it, if all you have gone over by is that much, a few days of lower syns will balance you out with no harm done.