1.25 oz Gin / 0.75 oz amaro /0.75 oz sweet vermouth / 0.25 oz maraschino liqueur / 2 ds orange bitters
I don't know that 0.66 oz (20 mL) will be all that different than 0.75 oz (22 mL) - perhaps the 20 mL version gives you more of a 50/50 drink (that is, 37 mL gin and 40 mL of the amaro/vermouth. I suppose you could go fully metric, too:
40 mL gin / 20 mL amaro / 20 mL sweet vermouth / 10 mL maraschino
Most of the recipes for this drink I've seen online seem to specify a London dry gin, but I think that an Old Tom gin would have been used when the original Martinez was made, and I think that Hayman's works quite well.
I made two of these last night with the recently released Cynar 70 - they were excellent! This is one of my favorite drinks, and I think I will be making it this way in the future. The increased alcohol content is not noticeable on the palate, but I did feel it.
Has a Black Lodge to start the evening tonight, so I thought I would try one of these with Michter's single barrel rye instead of the gin. It works but definitely not as good as the original - the gin gives a much cleaner flavor, the whiskey dulls the drink.