delay/echo is only for a simple delay, with a fixed delay time.
what you want is delay/write and delay/read...
delay/write you use to name create the delay line (with a maximum size), send in the signal you wish to delay.
delay/read (there are variations) the outlet is the delayed signal, the inlet is where to read from in the delay line, as a 'percentage'
e.g. if you take the delay/read with the blue inlet (control rate), if you pass in 32, (i.e. half a dial) it will read from 50% of the delay line, so if in delay write you have 512 samples, 50% is 256 samples.
obviously, msec can be calculated, as you know there are 48k samples/sec... and as the buffer is fixed in size, you can calculate what percentage of the buffer is required for a given delay time.
I'd guess, at this point, your thinking this is more complex than it needs to be... and yes, it kind of is, but that's because these are fundamental building blocks, from which you can build all sorts of effects, as I said its worth getting used to these...
but, I'm sure someone has wrapped these into something easier to use in the community library for the final use.