Rewinding a toroidal transformer...what voltages would be good?

[xenoxaos]

I've got a 115v to 27v transformer. I was wanting to rewind it so I'd at least have a center tap for +/-12v. It seems to be fairly beefy for my needs so I was wanting to rewind it for multiple uses. What output voltages would all of y'all recommend winding.

Should I go with center tap or just two windings I can separate/combine at will.

[Gyro]

Two windings halve the length of wire that you have to wind on at one time. 

[james_s]

When I've wound transformers like that, I wound a single bifilar winding to get two separate identical secondary windings I connected in series as a single center tapped winding. Half the amount of work as two separate windings and a lot easier to make sure it's identical which is important if you ever want to wire them in parallel.

[pqass]

Temporarily wrap 10 turns of insulated wire around the toroid, apply mains to the primary winding, and measure the 10 turn winding (leaving the existing 27V winding unconnected). Divide by 10 to get a volts/turn value. 

Now you can use the volts/turn to dial any voltage you may need.
If there is enough room, you don't even have to unwind the 27V winding; just leave it unconnected.

Do you know the power rating (in VA) of the transformer? 
If so, just make sure that any/all new secondary (or secondaries combined) don't exceed the VA (ie. the wire diameter chosen amp carrying capability multiplied by the voltage).
If not, use the diameter of the existing 27V wire as a rough approximation of the current capability (lookup dia->amp in table and multiply by 27V to get VA).

If your new secondary is wound to produce less voltage, you can increase its diameter proportionally to give more current.