is there a proper way to lean mixture on aircraft like DHC-2 Beaver or Grumman Goose? Those do not have EGT or Fuel Flow gauges, which is what I have read is used to determine how to lean mixture.

I can wing it (pun not intended) with 15% per 1000ft seemingly working fine, but would like to know he proper way of doing this.

Don't know about those specific types but on smaller GA types you would lean the mixture just to the point the engine begins to run rough, then richen the mixture a little until the rough running ceases.

As you lean the mixture, watch the tachometer.  Lean until the RPM begins to drop from its maximum, then enrich the mixture just enough to get it back to that maximum.

Those sound like good guidelines, thank you!