Munich is right on the threshhold of being able to convert itself, so you REALLY have to hit temps and PH to get it to convert the same way something like two row would. The manufacturer's numbers state that you'll get 79% conversion in 20 minutes of mash at proper temp and PH.
Some peeps will do like a 75-90 minute mash if they are using high % of lower diastatic malt.
Add just 1lb of two-row and you won't have a problem at all with the Munich. Otherwise, it doesn't hurt to add about 10% extra Munich to hit your #s, and watch the mash temp like a hawk to keep it constant, or just let it mash a bit longer.
Lots of options, but you get the idea - you are likely to come in a little low, so pump up one of the variables that contributes to efficiency.