Technically, the answer is infinity. If you keep going halfway, you’ll never actually make it to the doorway.

But in practice, after 10 steps you’ll be 1/8th of an inch to the doorway and anyone behind you would say to stop being a baby and take a real step through the doorway already.

A = 1
B = 3
C = 9
D = 17

1 lb = A
2 lbs + A = B
3 lbs = B
4 lbs = A + B
5 lbs + A + B = C
6 lbs + B = C
7 lbs + B = A + C
8 lbs + A = C
9 lbs = C
10 lbs = A + C
11 lbs + A = B + C
12 lbs = B + C
13 lbs = A + B + C
14 lbs + B = D
15 lbs + B = A + D
16 lbs + A = D
17 lbs = D
18 lbs = A + D
19 lbs + A = B + D
20 lbs = B + D
21 lbs = A + B + D
22 lbs + A + B = C + D
23 lbs + B = C + D
24 lbs + B = A + C + D
25 lbs + A = C + D
26 lbs = C + D
27 lbs = A + C + D
28 lbs + A = B + C + D
29 lbs = B + C + D
30 lbs = A + B + C + D

(7 x 7 x 7) + (9 x 9 x 8) + ((9 – 8) x 8) = 999
(77 + 7) + 888 + (9 + 9 + 9) = 999

If other operators were allowed, another solution would be 999 modulo (888 x 777). I’m sure there are more solutions.