. . . the front of the armhole is ok but the back is cut back further than I like so I have added a bit to it.
But.. the back of the armhole is gaping quite badly. . . .
Rather than the back armhole being cut in too much, I wonder if what you need may be some back shaping adjustments. The first thing it helps to do is work out what kind of shoulder and back shape your body is. Shape matters, not just measurement, because it's different for everyone.
Some adjustments to think about are are -
Shoulder slope and Shoulder seam position (ie. shoulder angle)Broad Back AdjustmentRounded Back AdjustmentCombination of Broad Back & Rounded back
It is important to check your shoulder slope and position compared with the pattern.
It helps to sort out the shoulder slope and angle before doing the back adjustments.
When doing any bodice fit adjustments, it helps to follow the order of -
1. Shoulder slope and Shoulder seam position
then generally -
2. Back (length & width & shaping) if required
3. Front
This Threads Article about fitting the armhole is a good read to point you towards the right direction of what your shape needs
Broad Back & more shoulder length required Adjustment 1
Broad Back Adjustment 2 This doesn't alter the length of the shoulder seam
If you have a combination of both broad back and rounded back, do the broad back adjustment first.This is a lovely (as it actually happens)
video by Joy of one way of trying to measure how much adjustment is needed for a full or rounded back adjustment.
Note - some of these full or rounded back adjustments create a dart (or make an existing dart bigger).
If you prefer to remove a new dart, the process is to rotate it out. There are plenty tutorials around about how to do that.
Watch this
Rounded back Adjustment Video from 1.30. It's often called rounded back and sometimes the dowager's hump adjustment but it's really about "fullness" across the back because we all have different postures, angles of shoulder blades and padding in different places.
Depending on where the fullness is on you, there are some different back shaping adjustments you can do.
This
example of a high back adjustment is a variation that places a dart into the neckline rather than the shoulder
and here is
Another example of a variationThis is another way to work with the fitting adjustments on a body with a toile/muslin. Different issues are demonstrated, but it will give you a clue about where the fine tuning adjustments will be. These adjustments do need to be done on a toile/muslin and then the adjustments done to the pattern before cutting out the real fabric.
Hope you find something useful to help you work out which type of adjustment(s) you will make for your shape.