you could always do it the old fashioned way:
When drawing the sub and target speeds.. use lines of the same scale.
If you will be travelling 20kts, draw your speed line either 200m, 2000m, 20km, 200km etc. etc.
If the target is doing 12kts, draw his line at either 120m, 1200m, 12km, 120km long.
Pick whichever scale you want, but use the same scale for both.