Cartridge alignment tool

Generally agree with brf.   

There is no uniquely best way to set the horizontal tracking angle (HTA) for a pivoted arm. This is different than azimuth and even SRA where in each case there is a best position (even if it varies by disk for SRA). Experts disagree quite a bit on what HTA rule is best. Each possible setting is a compromise. Moreover, for this reason IMO the effect of a different HTA is exaggerated by some people and I am always amused to read that someone spent hundreds and hundreds of dollars on a protractor and came up with an HTA setting that "blew me away." My own suggestion is not to obsess over this setting. 

Any of the usual tools used with care will get you to a reasonable solution.  The only advice I have is that an even inexpensive tool with a mirrored base can help avoid parallax error.

All these suggestions are good, but you should know one thing about the Mint - It requires you to line up the cantilever between two minuscule parallel lines and this is very painstaking and, IMO, virtually impossible on a cartridge with a small, tucked-in cantilever.  It's much better for carts like the Lyra with an out-front cantilever.  Just letting you know.  I bought one, but it was just too onerous with my Transfiguration cartridge. 
I just use the gauge that came with my Basis tonearm and it sounds fine.   

Though expensive, the Smartractor is the best. It saves a great deal of time, easy to use and has a unique alighnment called the UniDin (along with all the others). I love this alignment since you seem to get no perceptible IGD. I know it’s not inexpensive, but if you have multiple carts it saves you gobs of time. 

brf, thank you for your reponse, I think I would go for the MintLP, for my REGA RP6 / RB303 tone arm.  For the download printable protactor, it is the hole to make after the printing, that may cause errors ?

I agree fully with @last_lemming 
Nothing comes close to the Acoustical Systems Smartractor. For me, it is well worth the investment even if you only have 1 cartridge. I have typically only had 1 cartridge at a time except for the last few years when I have had 2 arms and 2 cartridges. It gets me the best result and that has made it invaluable to me.