Yea, 1:5 is probably rather higher than I would have chosen, but it was what I had available. While I have re-wound SMPS transfomers before, I've found it a bit of a fiddle (chiefly, dissassembling the original without breaking anything) so I wasn't going to if I didn't have to.
On the other hand, 1:5 has actually worked out better than I'd expected. It gives plenty of "headroom" for the regulation to work in - the power source is a 12V lead-acid, so could concievably drop to 10.5V or so under heavy load. Admitteddly, the regulation is actually pretty poor as it stands, but I'm sure that it could be tightened up (at least in theory).
The transformer I'm using (from an ATX power supply) would originally have been configured as 10:1, to produce 5V from about 170V... which is even moe lopsided! I suppose they must have needed that ratio to maintain regulation under worst-case line/load conditions.
Actually, I'm quite surprised I've not saturated it, given that I'm driving at probaly 1/4 of it's normal operating frequency - but it's showing no signs if it.