Blocking and chest measurement

Hi
Help-we are visiting our 1 1/2 yr. old grandson for Thanksgiving and I want to finish the sweater I have made for him (my first sweater). I want to block the pieces before seaming and my question is "how and where do I measure the chest measurement of the finished piece to make sure it is what the pattern says it should be?. The pattern calls for 26 inch for the chest. Do I measure both the front and back pieces and add those two together? And ,most importantly, where on the pieces do I start the measurement-where the armhole starts, a few inches up or where?
Thanks for any help you can give me.
Merrily

The 26" is for the front and back together. You’d want to block each piece to be slightly larger than 13" to allow for seaming.

I’d measure at the underarm. That would be the widest point of the sweater and the chest.