Questions about agitation....

I have been developing my own black and white film for about a year. In the last 2 months I have been using Agfa APX 400(which I got really cheap in a bulk roll) and HC-110 dil.b which I use as a one shot developer. After some reading I decided to change my agitation technique. Previously I agitated for 10 secs at the start of development, and then 10 secs every minute. I did this for 6 minutes with the above combination and got acceptable results generaly. The last two rolls I have changed my method: the first minute of development I used continuous agitation, the for 5 seconds every thirty keeping the 6 minute development time. These results have been not so good. Should I add a minute to compensate for the first minute of agitation? Or should I just go with what works?

You should just go with what works. The first method you used is the one Ilford recommend, and the second is the one Agfa recommend.
When you say "not so good", are the negatives over- or under-developed?

I use Ilford film and technique and it works fine. Agfa film may require more initial agitation or may not, but you need to do what works for you.

I believe in using the technique the manufacturer recommends. If the contrast is low, extend the time.

I recommend always go with "go with what works" for you. We all have our own rhythm and way of doing even the simplest of tasks. I use Rodinal for all films and usually fill and invert for the first 30 secs and then 1 inversion every 30 secs. But if I want more contrast I'll increase the initial number of inversions, less contrast I lower the initial number of inversions. That works for me but not everyone, I know some increase/decrease the number of 30 sec inversions.

Are the "not so good" results too contrasty, not contrasty enough, or is the new agitation producing less film speed? Basically, the amount of agitation is as important to controlling the degree of development (=contrast) as time and temperature.

A secondary effect of less frequent agitation can be slightly more film speed, and maybe less grain (particularly with a developer like Microphen or Acufine). My personal preference is for less frequent agitation, but I start with a full minute of constant agitation to minimise the chance of unevenness due to uneven uptake of developer into the emulsion (including air bells etc).

The negs have less contrast are kinda underdeveloped, I also should add I am using a Two reel tank with the soloution to cover one reel completly so when I invert the reel is not submerged.