From memory, the last one I did was an offset of 1/2", which is a very good starting point. As Ginge noted, the wheel can then be finally trued up in the frame because it takes very little tweaking of the nipples to pull the rim across 1/4" either way. However, I would true the two wheels together with a couple of parallel straight-edges for a perfect alignment because that's my method and it works for me. I always did that on race bikes where it can make a difference but a humble C will be fine with less accuracy. Just get it in line with the centre of the frame, check alignment with the front wheel and all will be well.
There is no point in fitting a bigger tyre than a 300, it's only cosmetic. I would be very surprised if any 350 went into a C plunger frame because I could only get a 325 Dunlop into my plunger C11G by running it on the piss to clear the chain guard until I got a new 300 to put in. I know different makes of tyres are different widths but there's little point in going up a size of two because there's not enough power to gain anything and it puts alignment out when you lean the thing on different width tyres front and back.
It's illegal to run a bigger tyre on the front as the largest tyre(s), if they're mixed sizes, has to be on the back of any vehicle. In a nutshell, I'd fit a pair of 300-19 tyres on the bike, pressures 28 psi frt, 30 psi rear, - it will improve the sharpness of the handling no end. However, it's your bike so do whatever you like with it, Donald!
Ray
