You have 15 syns a day, if the cake was 13.5 syns then what is the problem and why do you have to be extra good for the rest of the week?
I see no panic at all, the good thing is that you synned it and know the syns.
All is well in my opinion
I'm with Jayjou no harm done, even if you went over your syns for the day you can flexi-syn and there should be no harm done.
Instead of looking at how 'naughty' you were when you weren't, you should focus on how fantastic you were for having only the ONE slice, and also then working out the syns and sticking to plan!!
Pat on the back needed for tara40 for making great choices when many would have blown it big time. But then a slap for not realising how great you are and managing to see negative where there really is only positive!! lol