Come on Jean, these are you're own numbers. And looking back I did not misread the numbers, they are in your post
<motor port="1" rotation="+1">+0.243, +0.216</motor>
<motor port="2" rotation="-1">-0.243, +0,216</motor>
<motor port="3" rotation="+1">-0.244, -0.244</motor>
<motor port="4" rotation="-1">+0.244, -0.244</motor>
</geometry>
This is the start.
So you have an positive X as of 0.243cm and a positive Y ax of 0.216cm
Replace that by this calc:
srtq 2.0/2.0 * distance, in this case sqrt2.0/2.0 * 0.243 and sqrt2.0/2.0 * 0.216 and so on.. (sqrt2.0/2.0 = 0.707)
The only reason its complicated and thus using the custom config is because it is just not a real X-config. The X-config will calculate the correct numbers but the custom config not.