CuttySark, It's kind of complicated and depends upon your individual circumstances - length of service, etc etc.. I'm not sure that there is an easy way to calculate the proportion of the commuted value that would be transferred into a LIRA other than getting your Retirement Benefit Statement from your employer upon your retirement.

The statement will show several options for life pension guarantee (with varying joint survivor percentages) along with commuted value.

In my case the commuted option was 'A transfer of $xxxxx to a locked-in retirement savings arrangement. The amount of transfer exceeds the maximum tax-sheltered value by $yyyy.. this excess amount will be paid in cash. 
For me the $yyy was $383,822 so one can take a bit tax hit. I took life pension with 75%JS. Fully comfortable that this was the right decision for me.