HELP: How many cans of R134a?

[onedumslack]

a little off topic but

hello
i am topping off the R134a in my 2010 audi a5

on max ac, with recirculation, 90 degrees F ambient, the center vent cant get down to 39 degrees F. which is great
however, the low side pressure only gets up to 28 psi. the chart indicates that the low side pressure should be 45 psi.

hence: low side pressure too low, but center vent temperature ok

question: does the system still need more R134a?

If you're not having a temp problem then I would not add any more. Psi is relative to the ambient temp outside. There is no set pressure unless you have a set temp. The only way to tell you have the correct amount without calculating for temp is to empty the system and meter the correct quantity back in.