As we know, the custom is to cast on 10% fewer stitches for the welt than the body. Or, to put it another way, we have to increase by 10% of the total when we reach the end of the welt. (If your gansey is going to be 400 stitches in the round, cast on 360 and increase by 40 after the welt.)
Take the number of stitches you have to increase by and divide it in 2, so you ensure you’re dividing evenly between the front and the back. (If you have to increase by 40 stitches, that’s 20 for the front and 20 for the back.)
Then all you have to do is ensure that you’re dividing evenly across the width of the gansey – this is important because your pattern will look odd if it’s not centred properly, and you can only ensure that happens if the increases are evenly spread. (For example, suppose your pattern calls for a diamond right in the centre of the chest. If you’ve made all 20 of your increases close together on one side, the diamond will be off-centre and shame will be your invisible companion ever after.)
I find the simplest way to ensure an even distribution of increases is to count the number of ribs I have in the welt, and match my increases to a given number of ribs. So, if I have 20 increases to make on one side, and I cast on 360 stitches for the welt, that amounts to 90 ribs in the round, or 45 ribs on each side: 45 divided by 20 equates to one increase every 2.25 ribs (or thereabouts – you don’t have to get too hung up on this; approximately is fine).
And because, as I have said many times, I can’t count stitches to save my life, I keep track of each increase on a piece of paper using the 5-barred gate technique, ticking each one off as I go. Just do whatever works for you.