I-485 applicants often have been fed with either wrong or incomplete information on Birth Certificate information.
I would highly recommend you to get it from the District Office (DO) from where you were issued Nepali Nagarigta-ko Pramaan Patra. They will issue it in Nepali language. This document should bear Your Name, the Name of your Father and Mother, Date of Birth (in B.S.), Place of Birth. Sometimes, the DO might require you to prove your relationship of being a son or a daughter to your parents. To save time get prepared with recent Nepali Passport size photos, a copy your Nepali Passport and a copy of Nepali Citizenship document sent to Nepal. The DOB (in AD) on your Nepali passport will be used on your translated DOB certificate by the Notary Public in Nepal. Notary Public accepts scanned and emailed copies of your PP or Nagarikta.
Your SLC certificate or any other certificate would turn out to be inadequate because most of such documents do not spell out your Place of Birth or Name of mother and the likes.
Once you receive the DOB in Nepali and a translated version, send copies of those and not the originals to the USCIS.
Disclaimer: This suggestion is based on experience and should not be construed as a legal advice. Do your due diligence. To clarify specific question, you might wanna call the USCIS toll-free number and/or see an immigration attorney.
Good luck.
