At least one manufacturer of a wind trainer in the 1980s suggested that its resistance at a given wheel speed wasn't far from that experienced by a road rider on level ground; i.e., however fast your were able to ride on the road in a given gear, the same cadence and gear would require about the same effort on the trainer.

For your setup, probably the simplest approach to resistance calibration would be to use a friend's smart trainer and do some steady-state efforts while keeping track of cadence and power versus heart rate. Although direct power measurement tracks instantaneous power changes and heart rate measurement does not, heart rate tracks power output quite closely over steady-state efforts.

Then, you can buy a cheap heart monitor (or a bluetooth or ANT chest strap that transmits pulse data to your smart phone) and a cadence sensor (one that also connects via bluetooth or ANT) and simply watch your heart rate. If, for example, pedaling 90 rpm on the smart trainer while putting out 250 watts put your heart rate at 150 bpm, maintaining the same cadence and heart rate on the wild trainer would mean that you were putting out 250 watts.

From what I've read, respiration rate and perceived effort also track power output reasonably accurately. And after all, those two factors were essentially all we had to go on before the advent of electronic monitoring.