As far as I can see I think you are getting confused over this 'intervening rest'. The idea of 'Standby' is that you go from it to a 'duty' without any 'intervening rest' (this would almost certainly be 'min rest' or more, in which case your standby finishes when you are notified of this 'future duty').
Otherwise, once you commence the 'duty' from standby you are limited to the max FDP as per EU-OPS Pt Q (basic 13hrs +/-) extended where applicable by either suitably provisioned split duty rest or by 'proper rest' - as you were- in which case you then re-commence a new FDP for your return sector in the morning, and yesterday's standby is 'history' and irrelevant. Your return sector is a whole new duty, subject again to the basic 13hrs.
The duty you quote would be one most pilots would kill for from standby!
