How to choose the right protractor for my effective tonearm length

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!

The Conrad Hoffman Arc protractor is free, customizable, dead accurate, free, easy to use, only 1 point to make parallel to, free, and you can choose your radii for standard DIN, JIS, IES, RIAA, and choice of 3 alignments. 

Line up to the arc, make parallel, done.


Did I mention it’s free?

On Vinyl Engine under Cartridges and preamps.

I also have to endorse the MOFI Geodisc. It is a very quick and easy tool to use. I have other alignment accessories in my toolbox such as my recently acquired Wally tools, Avid glass protractor, and from Germany, the Schon protractor.

In my experience after many years in this hobby, you tend to use the most time efficient method to dial in your cartridge.

I mean, who wants to spends hours and hours aligning such a small needle in an arc with these other tools. But, after you gain experience, you yawn when setting up cartridges with the MOFI, and perhaps, may want to explore using other tools for the thrill of the chase, or to genuinely compare other tools approaches to achieving alignment success.

Ultimately, your ears also play an essential part in getting anything right.
Once you go into the deep end, like I did, Not only do you have a tool kit full of other alignment tools for the sake of and because you can, but the Fozgometer azimuth tool ultimately ends up as part of toolbox as well. It’s all part of the hobby and in any hobby, you have support accessories to help you enjoy that hobby. That’s my .02 cents.

Dear @peterhaze476 : "   I currently have a Baerwald standard protractor that was made in Great Britain...... ..."

That's all you need for both tonearms and follow the @lewm  advise on tonearm effective length.

In the other side:  ""  And have a basic mirrored Stevenson protractor. " "

Do it a favor to you and the quality room/system performance levels and stay away from Stevenson. You don't need that ST protractor, its best place is in the trash can.

Regards and enjoy the MUSIC NOT DISTORTIONS,
R.

Thank you for all the good info to all the responders. I will try out these protractors mentioned. Although the question I'm really trying to ask is how do I know that a protractor is right for my turntable other than the advice of anyone telling me use this protractor or that protractor. How does any given protractor achieve the correct over hang when not all turntable over hangs  or tonearm lengths are the same.