Off the top of my head (I've written it somewhere else on here) the Haynes manual recommends 80Nm for worn belts and 110 Nm for new V-belts. To achieve this get a torque wrench that has a dial on it (not one that "cracks" as it reaches its torque) insert the head into the drive hole on the side (the big square hole that looks like the back of a socket) and push until it reaches the desired torque. Then do everything up.
I tried this and it's not very easy. They need tightened all the time and are a pain in the arse in my opinion and should be swapped for another set up at the earliest opportunity. There's toothed belt designs, v belt designs and serpentine belts. I've no experience of serpentine belts on a G40 but I'd guess they're better than twin V-belts otherwise manufacturers would use them instead of serpentine belts to run PAS, AC, alternators and whatever other auxiliary bits are bolted onto modern engines.
I find I have to adjust my set-up every 2 weeks or so (I run 65mm toothed belt) which is roughly 1000miles. I stripped 25% the teeth off the first belt and killed it in about 5k miles. But I do drive like a loon and am addicted to 3rd/ 4th gear overtakes