To get one piece of UU Matter your looking at 1 million EU! But luckily scrap can increase the efficiency of the Mass Fab. Scrap comes from recyclers which produce 1 piece of scrap for (roughly) 8 items/blocks. The best source for "scrap fodder" is a cobble generator. Some people macerate and cook the cobble to make glass for BC pipes.

Why bother with pipes? The main issue here is the speed at which you can recycle items not the speed at which you can generate cobble. Adding a Macerator just slows things up and wastes resources/power that could be spent on recyclers (and the Mass Fab itself).

The formula is something like this:

R = EU/18.86

Where R is the amount of recyclers required and EU is the amount of incoming power.

R must be rounded up.

(You need 1 recycler per 17.86 EU plus 1 EU/t for each recycler)

Assuming your pulling in 128EU/t you would need:

R = 128/18.86

R = 6.787

R = 7

Obviously if you are using solars you need to calculate night as well.

According to the wiki you get power from solars 46.8% of the time (I may have screwed up here it could be 53.2%). So you only need to be running your recyclers at 46.8% the rate.

6.787 * 0.468 = 3.18

Or 4 recyclers. Providing you have the storage space:

The recycler takes 45 ticks an operation. The 4 recyclers are thus generating 1 scrap every 90 ticks. There are (by my calculations based of the solar stuff) ~14835 ticks in a night so you will make about 164.8 scrap a night which is less than 3 stacks of space.

Obviously this guide is assuming your generating 120EU/t but I'm sure you can adapt it for your own needs!