Had a look at this, the thirds exercise for example, I can’t see how this is useful to be honest.
Memorizing letter combinations is a bit of a false dilemma.
Think of it in this way, do you really think: I’m playing a Bb when you improvise?
If so, you have to do that in all keys, and modes, which is madness, unachievable.
To think; I’m playing the third instead (for example) is related to the shape, which is moveable, would be so much better.
The cycle of thirds exercise for example, which Nal linked to, to do that inside the shapes would be a great exercise (it’s just arpeggios), but to be able to name the notes without a guitar, it’s a bit pointless.
I mean, what are you going to use it for?
Intervals is everything, to translate them to letters should always be secondary.