Hi Roaminoz. If your post is already in the ground Just cut your...

Hi Roaminoz.
If your post is already in the ground Just cut your rebate all the way through the face of the post and let rail hang through and cut rail flush with the post, it will be a stronger joint especially if you are swinging a gate from that post, you will be able to get a better fixing though the rail into the post.

cramp a fence/guide to the post for your baseplate on the powersaw to guide your saw on, set the depth of the cut and run it strait through a number of times, You will find it will be a better quality joint and you will find if it is a narrow post it will very hard to chisel when your got nothing backing the post up.