bbt expert's, i need some advice!

I know its best to use a bbt thermometer to get more precise readings. but i also read you should stick with the thermometer you've been using to get accurate readings on your bbt...

so my question is, I've been using just a regular thermometer to measure bbt, my chart seems to be just straight mostly, a few(like one or two) dips and jumps but.... mostly straight across.... is using a regular thermometer doing this? should i add 0.5 degrees to each of my readings if im using a regular thermometer?

will getting a bbt one mess up my readings since ive been using a regular one?

input? advice? thanks ladies!😄