Your math on the hive is incorrect. There are 4 factors that matter.
1.) Pop of planet = P
2.) Base bonus per factory (tech level) = F%
3.) Average adjacency bonus per factory and hive =A%
4.) Number of factories on the planet = F#
The difference in absolute production hive - extra factory is:
You can show mathematically that the average adjacency bonus benefits the hive scenario as it increases. You can also show this for the number of factories already on the planet. The more that exist, the less the relative boost from another factory compared to a hive.
The only factors that benefit the factory scenario as they increase are the base factory bonus rate and the base population. At a factory rate of 50% the break even points are as follows (note this also assumes a 0 average adjacency bonus which is pretty much impossible):
Existing Factories - Break Even Pop
0 - 3
1 - 7
2 - 11
3 - 15
4 - 19
5 - 23
6 - 27
This should obviously make sense since the hive contribution is fixed per existing factory (4×0.5) while the extra factory contribution scales with pop. The fixed step per factory is 2 and the scaling portion per pop is 0.5 so 4 pop to break even...you just push the specific break even point along.
IF you do the same calculation but with an average adjacency bonus, however, everything changes. If we assume just a 15% adjacency bonus...which is pretty easy to gaurantee...these are the results for the same situation.
0 - 4.something
1 - 13.something
2 - 29.something
3 - 60.something
4 - 145.something
It should be obvious that this keeps getting worse and worse. The point here is that the adjacency bonus on the hive also affecting raw production is a huge multiplier. We aren't even discussing obvious other points such as:
1.) Getting higher tech factories costs research and build time
2.) Factories only help production whereas the hive contributes towards whatever you want the planet to specialize in
3.) The hive also provides adjacency bonusses to anything else it is next to like farms etc.
The importance of this building should not be underplayed. It is ALWAYS use full and even MORE useful at the critical early stages of the game. It will virtually double the speed at which you get your early research planets covered in laboratories and ready to roll. And all early game advantages end up yielding compound benefits over time.