For the Millipore, it's hard to say - for an IP, 5 ug could be the amount that they used to pull down the protein. Westerns are normally done using a concentration or a dilution. However, your calculations are correct, if you want 5 ug/ml, then 5 ul of a 1 ug/ul solution into 995 ul will give the correct concentration.
The dilution to use is normally determined by titration - take a sample that you know expresses the protein of interest (this could be a cell line, expressed protein from bacteria/yeast, purified protein etc), and a negative control, run in pairs on a gel, blot, slice membrane into strips so that each contains + and - samples, incubate each strip with one of a series of dilutions (commonly 1:100, 1:200 or 1:250, 1:500, 1:1000, 1:2000) , look for the one that gives the best signal, with the least background.