Carbonating with corn sugar in keg

I cannot find the answer to my question despite searching the forum.

I recently brewed a blonde ale small batch. 2.5g, OG 1.048, FG 1.008 with Wyeast1056. Racked onto corn sugar/sterile water simple syrup in sanitized keg. Topped with CO2 and sealed.

This is my first attempt with keg conditioning.

I used BeerSmith to calculate corn sugar carbonation amount for roughly 2.5vol of CO2. It called for 2.5oz of sugar. After I put the keg away, I noticed that if I changed the calculation to "keg with corn sugar" it only requires 1.17oz of sugar. Did I overcarbonate because the keg is more airtight? or does the calculation reduce the required sugar because of some Co2 force carbing the beer when it is connected to the tank?

I only wanted to try this in order to limit use of Co2 for only pushing the beer through the kegerator rather than force carbing.

Any advice, thoughts are appreciated.
ipe:

You should be fine, good news is that if its over carbonated you can just purge the keg. I prime my kegs with corn sugar an never use less because its in a keg

I would think you might need more sugar to create more Co2 to fill the 2.5 gallon void to make enough pressure to push down in to the beer. I don't know if there is a calculation, if you already calculated, or if this is just my theory, but to me it seems with that much head space in the keg you arnt going to get the carbonation that you want. Maybe someone will chime in with some real information.

What I just said will all be irrelevant if you are using a 2.5 gallon keg. I am assuming you are using a 5 gallon keg.

As I've understood it, carbonating in a keg with sugar requires about half the amount that would be used to carbonate that same batch in bottles. I would suspect that you will end up with more carbonation than you've expected (almost double) BUT you can easily vent off excess co2 from a keg so not a big deal.

Thank you all for your help. I think I can chalk this up to an experiment. With the excess head space, perhaps the extra sugar will help fill that space. I did top off the 5g keg with about 20psi but it would probably get absorbed before the sugar turns to CO2.