Hey guys currently riding a 2012 Cervelo R3 size 54. Looking at picking up a felt FC frame set. Cant figure out if i should stay a 54 or size up to a 56 since the head tube is significantly shorter. I am 5'10.5 so i can go either way. On my cervelo i have the dust cap and one 5mm spacer under my stem if that helps. Thanks for the help.

Felt-http://www.feltbicycles.com/USA/2013/Road/F-Series/Frame-Kit-FC-.aspx

Cervelo-http://www.cervelo.com/media/docs/R3-f4f2ba9b-e376-4b93-9a11-9ce57652f024-0.pdf

You are 5'10" and what are you doing on 54cm Cervelo anyway????
Remember, short top tubes are invented solely for the purpose of people
buying bikes after test ride on adjacent to bike shop parking lot.

what are you looking to gain by changing? They are two of the biggest contrasts in geometry. What length is your stem on the Cervelo? What angle? What bars are you running?

Assuming you want to replicate your current fit:
As you pointed out, if you go 54 on the Felt, you are looking at ~3cm of spacers.
If you go 56cm on the Felt, you'll want a stem like 2cm shorter.