I used a litre because I only did the flat parts of the canvas, didn't bother with the sides or the back as the water runs off. I didn't even bother doing the bit over the cab as it's slanted and has never previously leaked on mine, but will be doing it soon as these days it's occasionally parked on slanted ground at just the right angle to make the bit over the cab level, although water's only leaking through on the passenger side at the moment. When it's parked on the flat there's no leaks at all.
The bits over the load bed used to leak loads though, climbing a step ladder, throwing the fabsil on there and crudely spreading it out with a gloved hand took 10 mins, initially I only did the rear half as an experiment and the leaking stopped there instantly and carried on on the bit that I didn't do, so that bit got done pronto. That was 4 years ago and the load bed's been dry since then, even when the truck's sat there with a few litres of water pooling on the canvas. Birds (feathered variety) have even used it as a bath but it's still dry inside. I've improved the strapping now to stop the canvas sagging so the birds have had to find somewhere else to bathe!