Alternative to a torque wrench

Gone.

I have a problem with the picture............The Spanner is the wrong way round

If your pull gauge is accurate, yes... it's a good way to check your torque wrench. The formula is simple enough. Weight times arm equals force. Weight is the pounds of pull on the gauge, arm is the distance from the center of the nut to the attach point of the pull gauge. For instance... say "a" in your diagram is 10" and you pull until you gauge says 20lbs of force (weight), then you have 200 inch pounds of torque (20X10=200).
For foot pounds, take the 10" and divide by 12.
10 divided by 12 is .8333.... times 20lbs of pull equals 16.666 foot lbs of torque. Or just divide the resultant inch/lbs by 12.... you get the same 16.666 ft/lbs.