Universal method for timing belt tension

So I did the timing belt on my 2004 Renault trafic. The engine is a f9q, 4 cyl sohc diesel engine. Apparently it skipped a teeth a couple of kms in. Fortunately it didn’t break the engine (which actually runs relatively smoothly still). It only comes at half a surprise because I tensed the belt pretty much at random, since I don’t have the Renault specific tool.

I will now retime the engine, but I’ll run into the same issue if I don’t tension it properly. What’s a sure fire way of tensioning a timing belt that will work most of the time?