There is a recent post in Ask an Apologist asking what week we are in for the Psalter. Fr. Serpa gave an excellent resource for finding the current day/week but what if it should become unavailable?
Here is the way I figured out: Remember simple division that does not break down to decimal places but keeps the "remainder" as a notation? Simply divide the week number (pay attention to this at Mass) by 4 and look at the remainder. 3 weeks out of 4, that remainder is exactly the week to use in the Psalter. Whenever there is no remainder, use week 4.
Last week was the 15th week of Ordinary time. 15/4 = 3 R*3* (3rd week in the Psalter)
This is the 16th week of Ordinary time. 16/4 = 4 R*0* (4th week in the Psalter)
Next week will be the 17th week of Ordinary Time. 17/4 = 4 R*1* (1st week in the Psalter)
If you must use a calculator, you can look at the place after the decimal, but you'll have to memorize the interpretation.
.25 -> week 1 in the Psalter
.5 -> week 2
.75 -> week 3
(no decimal) -> week 4
15th week: 15/4 = 3*.75* (3rd week in the Psalter)
16th week: 16/4 = 4█ (4th week in the Psalter)
17th week: 17/4 = 4*.25* (1st week in the Psalter)
Hope that's simple enough. If not, then the only advice I have is: don't get scrupulous about it.