My best guess would be to mix transparency into each color in proportion. Eventually you'll become better at knowing your inks and your papers, and your end result will look similar to your original.
In a design, you have a red square that is at 70% opacity and a blue circle that is at 20% opacity.
You fill two cups, one with 100ml of your red and the other with 100ml of your blue.
You could decide that you add 30ml of transparent solution to 100 ml of your red. to follow proportions, you would add 80ml of transparent solution to the blue.
The end result might be "offset" from your original photoshop, but colors will keep their "relation" to one another and that is pretty important visually.
(And for finger tests, I would say finger tests are way more sketchy when you're dealing with transparencies, but they can always be a good test while you're mixing your colors.)
You will eventually get good at this and get results closer to your photoshop original, but I'm sure it'll still look good while you're learning ;)