I'm trying to use lookup~ to create a mapping for a variable pseudo-doppler effect on a panner. It isn't working

I'm having trouble pointing lookup~ to the correct area of the table I want to use - telling it the sample offset and the table size. I want the lookup to remain centered on the buffer - the buffer is 4410 samples long so I want it to centre on 2205.

So I'm calculating offset based on the desired wavetable length - if I want wavetable length of 400, centred on the middle of the buffer then offset should be 2205 -(400/2) = 2005, right? Doesn't seem to work.

I'm feeding lookup~ with signal values from -1 to 1... so I think I got that right. I figure must be misunderstanding something quite fundamental. Any ideas?