In order to use a micrometer to establish your pressure limits you have to establish a baseline for measuring.
Get a box of factory loads, fire each and measure the case expansion about .20 inches from the end of the case. Record these and do an aveage.
Measure each twice, the second time at 90 degrees.
It is important that the brass you are doing this with is of the same brand and lot number as you are reloading. You can not use these measurements for other lots of brass or other brands.
You can then use these cases to develop a load increasing the powder charge by small increments. When the case head expansion exceeds the baseline you have established with the factory loads, you are very likey exceeding pressure limits for the brass and the gun. Back off your load.
Rather than do this I use a Chrony and start with a minimum load. I work up in .3 grain increments loading five cartridges with each load and putting each load in a zip lock bag with the date, bullet, powder weight and sequence number. I then start firing five shot groups over the chrony. I will use banquet paper rolled across the target board with 1" pasters for shooting groups. I will sometimes prepare this paper at home and number the target patches according to the number on the ziplock bag. This gives me a record of group size. Sometimes the most accurate load is a few grains of powder under maximum.
And I never exceed the max velocity suggested by the loading manual, even if I reach max velocity at a few grains less than they have. The consistancy of compents like primers and powder changes. Much of the data was developed in the 1950s, 60's and 70's. At max loads a diferent lot of primers or powder can make a five percent or more change in pressures.
When you are using max loadsand start with a new lot of powder, or primers it is best to back off five percent and check velcities with a chronograph. Also check group size.
I avoid max loads if I can for a number of reasons, throat erosion, noise, recoil, shortened brass life, not to mention they are not fun to shoot.
In a hunting load and in target shooting bullet placement is far more important than squeezing the last bit of velocity out of a bullet.
Wait until you get a chrono before you get really carried away experimenting. It will help with simple things such as lot variations in powder for example. 
