How to choose the right protractor for my effective tonearm length

I have a thin metal ruler, mm one edge, inches the other. 12" or 305mm.

I drilled a hole/notch one end, so it fits the spindle, starts at zero. just swing it around, pick a distance and location.

reminder: consider parked position clearances when dust cover is on when picking the location.

as for overhang, that ruler can be used for that as well, just swing it around the spindle.

the alignment discs are good for null points, overhang, and I use mine for anti-skate adjustments

https://www.ebay.com/itm/Turntable-Protractor-Alignment-Mat-Phonograph-LP-Phono-Cartridge-Stylus-A-N...

they make specif ones for specific arms, but this works for anything

Feickert is fine for your needs.  But so too are many other choices. By the way, the 223.5mm and 215mm parameters are the pivot to spindle distances recommended for your two tonearms, not their "effective length".  Effective length = pivot to spindle distance + overhang.

My favorite is the MoFi Geo Disc. Makes alignment so easy. The purpose by the way is to align the cartridge at one point in its arc. A longer arm reduces tracking error across the record but makes no difference in alignment at that one point.

I use DBP-10 for so many years and is excellent, does not cost much either. 
G

As long as your tonearm was mounted correctly with the pivot to spindle distance right on the Baerwald protractor will do just fine. You have to align the cantilever arm perfectly parallel to the lines not the cartridge body, with the stylus right on the cross. With good light you get right in front of the cartridge with your nose almost on the platter and line that cantilever right up
petg60 is referring to the DB Systems protractor. It is an excellent protractor for very little money, $45 I think. Spending more than that is IMHO a waste of money. The single greatest source of error is the operator. Not the protractor. You have to take a deep breath, get your fastidious hat on and tweak that cartridge into position. You have to leave the screws just loose enough so you can move the cartridge with just a little resistance. Once you have the cartridge in position you tighten each screw just a little at a time going back and forth until they are just snug. The result of overtightening is total looseness!