Does the rating conversion make sense?

I was trying to work out what my elo rating used to be.

I had a BCF rating of around 115-120. I would play about half way down the boards for our chess club's second team and in congresses I would be about 30 to 40 percentile.

I used the following formulae to convert to elo

7.5 x (old grade) + 700 = new rating.
7.5 x 115 + 700 = 1,562.5
7.5 x 120 + 700 = 1,600

I didnt play chess for 35 years and have just started again on chess.com. I'm 73.

I used to play at a speed of about 40 moves in two hours but now I am playing my games in 10 minutes. It was a shock to the system but I have adapted.

I have read that chess.com grade and elo rating are pretty similar.

I would be very happy with an elo rating of 1,500-1,600 but my chess.com grade is 400! It seems reasonable given how I do with random opponents of similar grades. I dont really notice anything drastically worse than when I used to play before but I could accept not being so good but only 25% as good is a shock!

Is the conversion I am using correct? Are there other factors to take into account.

Online chess is very different to OTB, probably due to the time constraints. I don’t think there’s any formula cuz there’s so many types of ratings (JCL, FIDE, etc)