Something is wrong with the description of tinkerer

I have an outpost with 120 max shield (20 shield + queen + 4 kings). My tinkerer is there as well, and the outpost currently has 11 shield. When I scroll forward in time, I find that the outpost stays on 11 shield, that is, the outpost recharge is entirely cancelling out the -3 shield charge by the tinkerer, however if you do some calculations:
+120 shield in 48 hours = +120 shield per 2 days
-3 shield every hour = -144 shield per 2 days
Therefore the net charge is -24
However the shield charge is just staying at 11 and not moving at all, and whats more, if I move one or more of my kings from that outpost, the shield starts decreasing again...
My initial thoughts are that the tinkerer actually has a -120 shield charge per 48 hours its just that when you round it you get -2.5 per hour which rounds to -3, but maybe the Devs didn't want to put a -2.5 because it looked bad?
Anyone else experienced this?
This would also answer the question about how much shield charge you really need if you wanted to have a tinkerer without any net loss of shield...
+120 shield in 48 hours = +120 shield per 2 days
-3 shield every hour = -144 shield per 2 days
Therefore the net charge is -24
However the shield charge is just staying at 11 and not moving at all, and whats more, if I move one or more of my kings from that outpost, the shield starts decreasing again...
My initial thoughts are that the tinkerer actually has a -120 shield charge per 48 hours its just that when you round it you get -2.5 per hour which rounds to -3, but maybe the Devs didn't want to put a -2.5 because it looked bad?
Anyone else experienced this?
This would also answer the question about how much shield charge you really need if you wanted to have a tinkerer without any net loss of shield...