That’s because the number of 290 hp/s isn’t entirely from your shield booster, so it doesn’t get boosted entirely by the bastion and amplifier.
See here an unfit Golem with 16 hp/s passive regeneration:
This means your shield booster base repair rate isn’t 290, it’s lower by the amount of passive regeneration you have, in my case 16.
(290 - 16 ) * 2 * 1.4475 + 16 = 809. Pretty close to the number you got. Do you have some shield regen or additional shield implant?
Passive regen is not included in the value displayed for “Active Boost”
Oh you are right. I use the in-game fitting window too rarely to even know there was such a button. 
In that case I don’t know why it’s off. Some strange rounding perhaps.
Are you using any implants in this example v m x?
Only capacitor and missile implants. Nothing regarding shields at all.
I’m getting 317.6 hp/s base.
635.3 hp/s with bastion.
459.8 hp/s with amplifier.
882.3 hp/s with both bastion and amplifier.
317.6 * 2 = 6.352 → within rounding error correct
317.6 * 1.4475 = 459.7 → within rounding error correct
317.6 * 2 * 1.4475 = 919.4 → much higher than the 882.3 that I see in my screen.
You’re right, something is strange here.
Maybe Marauders 5 Golem bonus(37.5%) is in the play and is throwing us off
If that had stacking penalties with either of the amplifier or bastion shield boost bonus it would already result in calculation mismatches for either of them alone.
But those are correct, so that’s not it. Something about the combination of the boost from the amplifier and bastion is causing the total to be less than you would expect.
I think I know what it is. It’s just regular diminishing returns again. 
44.75% * 0.869 = 38.89% (diminished)
317.6 hp/s * 2 * (1 + 0.3889) = 882.2 hp/s
There we go.
I made the initial mistake to multiply the whole factor of the amplifier (1.4475) with diminishing returns (to 1.255), but that was a much harsher penalty than 86.7% of 44.75% (to 38.9% for a factor 1.389), so I came out too low and dismissed diminishing returns as cause. Oops.
How come bastion does not impact diminishing returns of hardeners(resistances) or ballistic controls(rate of fite) but impacts the amplifiers. Could this be classified as a bug?
Look and behold, a typical Newbro Vessel…
So if you are heavily neuted, your ship is helpless. I hope you have some ISKies for replacement.
But this is completely out ouf topic, the provided help is quite good.
Happy ratting!
As I said before, the bastion resistances are on a separate diminishing returns for resistances shared only with damage controls and reactive armor hardeners. So if you would equip a damage control together with the bastion you would get diminishing returns for it, but not for shield hardeners.
I don’t know how the diminishing returns with ballistic controls work of the bastion. My guess is regular diminishing returns, just like with the shield boost amplifier, but haven’t looked at it.
