The first and foremost rule of page layout is to forget the rules and do what looks best.
Try for pleasing proportions and then take a ruler out when you want to put the actual measurements into the word-processing document values.
As far as the top and bottom margin's concern, I use 1:1.62 (Golden Mean) proportions to start.
Left and right margin are set to allow optimal line length for the type size. There are various "rules-of-thumb" to follow from (1.) 8 to 10 words average, (2.) 65 to 80 characters per line average, (3.) the line length in ems equal to three times the type size in points, etc.
Once you have the optimal line length, use 1/3 the remainder for the inside and 2/3 for the outside margins.
Another suggested proportional margins for the text block are inside: 3 units, top: 4 units, outside: 6 units; bottom 8 units.
The pagination goes outside the text block.
There. It's beginning to look fantastic, already. Don't you agree?