The best way to measure speed quickly and easily if your using TDW's NewUIs, such as the current NewUIs_6_9_0_ByTheDarkWraith, is to bring up the conventional clock (L key) and right click on it. This will bring up the very accurate and historically correct U-Jags chronometer version which was actually used in U-boats.
To use this clock either begin your timing as the bow of the target passes your scope's vertical crosshair and stop it as the stern passes the crosshair or using the TAI map do the same with enough magnification to show the entire hull figure rather than just the contact circle or diamond.
What to do with the results of your timing are well shown in Stoianm's excellent tutorial video here:
How to use the U-Jags Chrono in SH5 ==> Link
The advantage of the method is that it only takes a few seconds, not 3 minutes and 15 seconds and you don't have to refer to any charts.