Check with your CPA, but I believe that a careful reading of the SECURE Act will show that the 10 year period ends on 12/31 of the year which includes the 10 year anniversary of the date of death.
What this means is that if your Mom died this year (2020), then you have until 12/31/2030 to deplete the account to zero. Which means you actually have 11 tax years, not 10. So I'd divide that $450K by 11, getting ~$41K. If your Mom died in 2019, then this doesn't apply - you'd only have until 12/31/2029 in that case.
With regards to earnings, my own strategy will be similar but somewhat different: I plan to take current balance divided by 11 the first year, then current balance divided by 10 the second year, then current balance divided by 9 the third year, etc. This would have the effect of spreading the effect of earnings over the remaining years of the term period, producing a smoothing effect.