ovulation testing with scattered cycles ?

Ra
Cycles !? When do I test. Bought ovulation strips today. Boots had a 3 for 2 offer on 😬
But my cycles have been 30, 23, 32, 21 should I assume my cycle this time will be 30-32 or go for the lower numbers. When should I start ovulation strip tests? 
New to all this. Never done an ovulation test before any advice appreciated. TIA 💕