Drawing revisions code?

Hi everyone! Hope someone can help me with this issue I've been dealing with for weeks now.

I have a column for drawing revision letter (A, B, C, D etc where D is the latest updated drawing) and another column for sequantial number that starts with 1 and is going to have updated rows under it with 1.1, 1.2, 1.3, 1.4 etc.

I hope there is a macro code that can run this sequence (when I click on Update Revision button it runs macro that inserts a new row WITH A as first revision and sequantial number (1.1), when I click for the second time I get a new row under the first one with B as my second revision and sequantial number 1.2 etc.)

My first row with sequantial number 1 has formula =IF(COUNT(G4:H5);MAX(G4:H5);LOOKUP(2;1/((COUNTIF(G4:H5;">"&G4:H5)=0)*(G4:H5<>"-"));G4:H5)) that returns the latest revision letter in the added rows underneath it. How do I do to make G4:H5 to extend and include all the added rows underneath it??