- 300 is a bad number. Try to round it to multiples of 8. I would say multiples of 16 but probably 8 will do.
300/8=37.5 decide if you need 296 or 304.
Define the size of the book. A tiny book will weight less than a humungous one, with the same number of pages.
Define the covers.
Weight alone is not the best measure to start with.
Do you want good print quality? You need coated paper.
Do you want lower cost? use uncoated paper.
Quality and costs are a heck more important factors than weight, but lets go to the math part.
Some basic math
There are several ways to measure paper. Let's use the international one.
A 150g/m2 paper says that each square m weights 150 g. If your book measures 50x50 each page it is 150/4 = 37.5 g each sheet... But a sheet is not a page, you have 2 pages per sheet, so you need 152.
37.5 x 152 grams per book. Now convert that into ponds.
That is the basic math.