Question on TSP cleaning

I have a few questions about cleaning with TSP.

- Various instructions I've read on this site suggest pounds per BTU input or pounds per gallon of boiler water. Which method is preferable to follow?

- If per gallon, how do I know how many gallons are in the boiler? I cannot find it in the specs anywhere; should it be on the boilerplate? (I have a Weil-McLain EG-65.)

- Should the boiler be filled to the top or to the normal half-way mark for cleaning? If the half-way mark, how does the whole thing get cleaned?

- The instructions on the box of TSP state that it can etch glass. I don't think I've seen any suggestions here, in Dan's books, or on the Weil-McLain web site to replace the sight glass after cleaning or perhaps shut off the valves to the sight glass during cleaning. Is this something to worry about?

Many thanks!