If memory serves, and it doesn't, we used to take the total cost of replacement and charge 10% per year for the age of the system i.e. if total system replacment was $10,000 and it was 5 years old, 10% = $1000 x 5 years old = $5000 for the year divided by 12 months. never sold one myself and never pushed them so I'm going by old hearsay

have fun
rich

(maybe it was 5%, sometimes I wish I still had all those brain cells that I burned off in my younger days)

"It's like the man said...never take life too seriously, you'll never get out of it alive" Bugs Bunny