ok (deep breath)

this was how it was explained to me 

the production from your moonstones is 1682 + 588 boost from the building which = 2270

you have 2561 gross production so your guardian gives gives you 291 moonstones extra per hour 

This extra 291 is shown in the gross production figures 

the calculation is this 

1682 + 588 = 2270 

2561 - 2270 = 291

therefore 291 is the boost the guardian gives you 

(thanks Katerina for the explanation) 

Thanks for the prompt reply Morteeee,

Interesting that the base amount for calculating the bonus includes the 35% for the Temple of Thea, I'll take it.

Guardian bonuses are therefor not separately indicated on a building or unit rollover menu.  Guess this makes sense as the other bonuses are permanent while Guardian bonuses are contingent on the Guardian remaining active in your city,  a highly variable proposition.