Here is a sodium phosphate buffer calculator.

https://www.egr.msu.edu/~scb-group-web/buffers/buffers.html

Here is a table, which you will have to interpolate for pH 7.7.

http://cshprotocols.cshlp.org/content/2006/1/pdb.rec8303

You may want to check my post at this analogous query:

https://www.researchgate.net/post/Why_do_I_struggle_to_adjust_lower_pH_of_my_phosphate_buffer_at_a_certain_pH_point_even_though_I_add_too_much_acidic_solution