If I understand you correctly, what you're wanting to do is to tap a tempo in and then lock that delay time in by setting the delay time to a millisecond value. Correct? There is a way to go about doing that, but it's going to be sort of roundabout. When you tap in a tempo, what you're actually doing is setting the BPM value for the scene you're in. To see this, go into the setup menu (on the M13, you hold down the looper button on the M9 you hit the 1B & 2B switches). In one of the screens, you'll see the BPM listed. You can then calculation the millisecond value. There's actually a chart on page A.3 in the Advanced Guide that lists these values for BPMs.

It's actually a simple calculation. For 1/4 note delays, the equation is 60,000 / BPM

For other note values, you have to think about what percentage of a whole note the delay time you're working with is. A dotted eighth note, for example is an eight note plus a sixteenth note, so that works out to .75 the quarter note value. An eight note triplet is 1/3 as long as a quarter note.

Or you could you just use an online calculator and forget about doing the math yourself! http://www.guitar9.com/delaytime.html