if you were to wait 30 mins for it to cool , you will have no dramas

as for water quantity, whilst you done to deviate ridiculously from tek, what is more important is having enough final liquid so that it is easier to pipette out

so lets say you using a narrow neck bottle ideally one with as long a narrow neck as possible, makes it far easier to pipete / baster out the laden solvent m, trade off is its slightly more awkward to get he ground MHRB into it in beginning

so use your 750ml as per the tek, and you can pour in more tap water to get it in prime position so that the solvent sits on top in the narrow neck

a few 100m or so more isn't going to drag down the PH of the base

you will find pipetting or using baster to extract solvent from a narrow necked vessel easier than just using a jar