Your solicitor is obliged to act on your instructions. I would advise postponing the meeting until such time as you are absolutely happy with the relevant figures.
I would suggest that you should not be happy with the figures unless they have been professionally quantified. Speaking for myself, I would never enter a settlement meeting for personal injuries/loss of earnings without an actuarial report. There is no hassle in obtaining a report - it is usually just a matter of days from initial engagement of the actuary to receipt of a report.
IN the circumstances, I would instruct your solicitor to postpone the meeting until such time as you have received actuarial advice. It may be easier for your solicitor to horse-trade with his opposite number on a ball-park figure, but i would suggest that it may be more beneficial for your mother to take things a little slower and to consider her position on foot of professional actuarial advice. This is particularly true where it appears to be a case in which liability is not contested.
As for attempting to work out the figures yourself, I am reminded of the old adage that "a lawyer who represents themself in court has a fool for a client". On no account should you bear the responsibility of calculating figures that are more properly the preserve of an actuary. You engaged a solicitor to deal with the issue of liability - now you should engage an actuary to deal with the issue of quantum of liability.
(Incidentally, I am not an actuary in case anyone thinks that I'm merely touting for business!)