Quick odd calculations

[ChipLeaderer]

I tend to calculate odds quite quickly usually rounding off. For example, If I have suited cards and 2 of those cards hit on the flop, then the odds of me getting my flush by the river is 2:1 against.

My reasoning goes like this: there are 4 suits and on the turn there's 4:1 chance of hitting my suit. Since there are 2 chances for me to hit my flush, the odds are 2 times as much, so 2:1.

Am I simplifying odd calculation too much and is there a long term loss if the numbers are rounded off to these approximations?