refill coolant ?

[Scotty]

Just had the 'brittle' upper coolant reservoir hose crack.
(same hose as here )

I have replaced the hose (2 min job), $32 part. Quicker to do this myself than flatbedding and waiting for dealer backlog !

I lost quite a bit of coolant as 'steam' and am refilling with 50% BMW antifreeze and 50% purified water.

The instructions indicate that you should fill with a cold engine; then run engine to normal temp. with the reservoir cap OFF (to assist air escaping)

Isnt this going to 'belch' a lot of rather hot air/coolant mist everywhere ?
Any thoughts on this from the knowledgeable here ?