>> it depends on your intended targets!
The point of that exercise was that you were designing an ASM for use against unknown targets, not least because they may end up being ships that haven't even been designed, let alone built, at the time you are designing the missile. Given all the other design constraints on any missile, it is generally better to minimise the MSP devoted to agility (for ASMs at least), because speed is a higher order determinant of hit chances.
However, since any missile has a base agility of 10, paying for 1 extra AG gives the maximum benefit in terms of hit %age to MSP ratio. Paying for an upgrade to 20 AG costs 10x as much MSP, whilst only doubling the AG stat. Paying to go up to 11 gets you a 10% increase in performance, for 1/10th the MSP.
Obviously, if you have any information on the targets you want to fire the ASMs at, you can make much better assumptions about what performance you need / want, but in the case where you just have no idea at all, going to 11 AG will usually give better average results for the smallest investment of MSP