Best thing to do is rotate the crank shaft by hand and when the piston reaches its furthest point away from the crank then that's your top dead centre which will be the same fore every cylinder. It should be anyway. The gauge should measure that and give you a figure. What do you need the tdc for anyway?
You need to know the tdc so it's in time with the rest of the firing sequence? Right makes sense. You probably don't need a gauge then really if you have a play around but there should be instructions on tinterweb somewhere