Hi Flound,
I think you're using the tool incorrectly. We've got a 35% cream here, that would have 2.2% protein, and a 3.3% milk, with 3.1% protein, so assuming that Scotsburn milk is something similar, then your f:p ratio would be 3.3:1! (or p:f of 0.3:1), so far more fat than protein.
Basically, to get that tool to work, you set all the "units" to 100, as per 100 ml (I do this because all the measurements on the milk here are things like 3.3g/100ml protein, etc; but if it said 5g/150ml, then the unit would be set at 150). The "amount" is how many of your units (i.e. if you put 100 for your units, then your 4 litres = 40; and your 1 litre of cream would be 10. If you had 150 as your base unit for your 1 litre of cream, then your amount would be 6.67 - much easier to adjust everything to a 100 mls. (If you're first cream was 10% with 2.2g protein / 100 ml, then the f:p ratio would be around 2.18:1 (or p:f of 0.48:1) so there's more fat this time, hence the increase shatter.
Anyway, I suspect that the extra fat from the cream is causing the shatter. But, you may have recovered, so all may be fine. Still, it is the ripening that will really determine how it turns out and while a bit of excess cream can help I suspect you will find your homemade bries will outshine any of the store bought ones even if you don't overload the cream into them. A lot of the butterfat will get lost anyway, if it can't be held by the protein structure, so you're probably not getting all of it into the cheese anyway.
Now, that being said, the proof is in the eating so if this ends up working well for you, who am I to suggest a change?