Calculate Moles of CH3NH2 Needed for pH 9.55: Buffer Solution

[alingy2]

You want to have a ph of 9.55 with a buffer. How many grames of CH3NH3ClO4 would you add to 800 ml of 0.258 M CH3NH2 to do this. Assume volume remains constant. Kb for methylamine is 4.2x10^-4.

My answer is 320g. Can anyone check this please? I have an exam on buffers very fast. You would do me a great favor.
I used Henderson to find concentration of methylamine required. Then the number of moles and grams.

 

[Borek]

I got 319 g, close enough.