You say the axle tubes are already shortened. Yes you can simply insert the original axle and measure how much extra it is sticking out. Just make sure you insert the axle all the way in until the end of the axle contacts the spider shaft. Obviously you want the axle flange to be located the same as stock relative to the end of the axle tube, when all the shortening is done.Can you simply not insert the original axle and measure how much extra it is sticking out?
Life is a learning curve. Even at my I am happy to still be learning new things. I will be first admit I have made mistakes along the way and take note just make sure that I do not make them again.I believe OP has it cut and is not sure of what was cut off.
I did as you suggest and took off exactly 1" and ordered 1" shorter axles.