New plaster and 50/50 watered down silk emulsion

If you are doing this as a mist coat to seal the plaster, then you have done the correct thing, for future it is better to use matt paint at a 50/50 ratio rather than silk, but this won't make a huge difference. You can buy out of the tin emulsions suitable for new plaster, but these tend to be overpriced and diluted paint works just as well.

it would be easier to buy a suitable emulsion for new plaster. Most decorating shops will stock one. I like Leyland matt emulsion for new plaster as the coverage is excellent. Why try and cut corners. I cannot tell you what it will do but i know it will not cover properly and the time you will waste is not worth the saving .