Use a fabric tape measure or marked 'calibrated' length of string.
Anchor the measure on the front instrument panel, extend it, by-passing the pilots torso - under arm, and secure on the seat / rear bulkhead (or fix it to self and sit back to hold the measure tight).
Note stick position as required, static datum (1.3 vs ?), thence record values relative to the datum.
Alternative, use an extendable tape measure with the boss secured to the stick top (double sided tape), but free to extent.
Attach the zero end to the front instrument panel and record measurements relative to datum as the tape extends vs stick position.
Voice recorder to note measurements vs speed.