You are probably doing fine, as 30cms using a scale of 1:50 is actually very small.
Looking at the image you sent of the different scales, look at the 1:50 one. This small line represents 1meter, or 100cm. Since you're stair riser is 30cm, it is going to be approximately 1/3 the length of this short line. Is that approximately the length you are drafting?
I dont know how to use the 'metric scales' ruler.
I have to use the scale 1:50 for drafting and the numbers on the rules are in meters.
i've been given the measurements in cm. for example, my stairs raiser is 30 cm.
You are working with a 1:50 scale
1meter = 100cm
Using the scale
1 meter is REPRESENTED by 100/50 = 2cm
Divide this by 20
5cm is REPRESENTED by 1mm
You could use this on your scale ruler.
Every length you have to draw, divide it by 5 and the length of the line you have to draft is your answer times 1mm
So your 30cm
30/5 = 6
You draft a line of 6mm
I hope this helps