Part of the issue is that if they have a quality second 3tech, then that player would play in the Nickel with Crawford and backup Crawford in the base. The second 3tech type player could end up playing as many snaps as the guy that is the 1tech starter in the base.
If they played 100 snaps on defense, then there would be 200 total snaps between the 2 DT spots
Crawford 80 (80% of the total)
1tech 50 (All snaps in the base, none in the Nickel)
Second 3tech type DT 70 (50 in the Nickel with Crawford and 20 replacing Crawford)
That converts directly to percentage regardless of how many snaps they actually play on defense:
Crawford 80%
1tech in base 50%
Second 3tech type player 70%
I say 3tech type player because sometimes that player is in the 1tech position on the field in the Nickel, but is not playing that same actual technique as the 1tech in the base. Sometimes they actually spread out the DTs and play them both as 3tech in the Nickel.