Multi Family & the 50% rule

[Bill J.]

Does the 50% "rule" have a sliding scale? Obviously an older property will require more repair and maintenance than a newer one would. Is it fair to assume for older properties I had better budget 50% of monthly income for expenses and I could budget 10-20% less for a newer property? Even at that, later years of ownership would require and increasing expense budget. 

Thanks in advance.