In many journals it is said to maintain
"81/2-by-11 double-spaced typed pages". This means how many inch margin?
That is what is the value of x in
if a journal requires "double-spaced typed pages" it is a certainty that they will reformat it for publication. as Werner says in his comment,
1in margins should be satisfactory in that situation. if they want something closer to their published format, they should provide an author package.
however, be careful to keep any display math narrower than the published page width, using multi-line display environments if necessary (cf.
reformatting math to fit within "smaller" journal page dimensions can end up introducing errors, never mind causing real headaches for production editors, even if the final conversion is tex-to-tex.