For AD&D 2E, monster THAC0 is 20- HD where HD is never even and rounds down to the previous odd number. If there are pluses to Hit Dice, THACO improves by one for every three HP. See pages 75 and 76 of the AD&D 2E deluxe DMG (the one DriveThruRPG sells.
So, yes, 20-49 means the THAC0 would be -29. Note in 2E a natural 20 always hits and a natural 1 always fails.
OSRiC or AD&D 1E is different in that the charts matter. You get several armor classes (six if I recall) where 20 will hit then it moves on to needing something that mathematically gets to 21, 22 etc. The charts on page 74 and 75 show characters top out at different levels up to 21 but monsters top out at 16+ HD with a THAC0 of 7.
OSR rules that follow the B/X D&D rules have monsters top out at 21+HD with a THAC0 of 5 per page 26 of the D&D Expert Rules (as written by David Cook, it changes).
OSR is a real mess as you can see. Blindly using 2E rules can add a lot of power creep to these edge cases.