Just curious: you are lucky enough to find an English speaking site where people who answer live in Japan and you post your salary via an exotic and confusing Indian system? Come back on Earth...
Regarding the 300,000Yen (or 30万円 if you need to show off), this is what you may expect
- 80,000 for the (cheap and small) apartment rent, including Internet, electricity, water, gas...
- 10,000 for extra transportation if you can stand living near Rakuten and go to the office by bicycle, then travel reasonably by train with rare extras to Kyoto [*unless Rakuten covers transportation fees
home-office ]
- 90,000 for the food and drinks - don't expect to survive long the
Matsuya and fast-food / bento daily. Food can be pretty cheap in restaurants, but as a gaijin you want extras. OTOH, strange enough, diy food can become pretty expensive.
(A lot of "Office Ladies" are paid ~180,000Yen a month, live 2h from work, eat bento all the time, and do extras once/twice a year max. Believe me, you don't want that life style.)
The rest - 100,000Yen in theory unless you have to pay taxes - could be saved?
I bet you won't save much...