I've been using 10 - 48 gauge strings for standard E tuning for 3 years, but I want to change to standard D tuning. I read somewhere that I should use 11 - 58 or 10 - 58 gauge strings for this tuning; is that correct? I don't want the strings to be too slack. And can I use one string gauge for standard D and drop B tuning ?
I agree with what you heard that if you drop all the strings two frets you want to go up at least one step in string gauge. Perhaps two. I played two frets down in one band and I ended up stringing 12s for that, so to me, 11s certainly make sense.
Drop B is a big difference for the lowest string. It’s gonna be floppy, but unless you get a custom gauge string, it will be floppy. If I were in your shoes I would just deal with the floppy feel and learn to work it. Another option is pick up a second hand guitar that isn’t too expensive and customize the strings for drop B and switch for those songs. A string that’s not floppy for drop B will be too tight for standard D tuning.
You don't say what the guitar is - Les Paul versus Strat means different tensions for the same string gauge and tuning, so it's not that straightforward.
It makes sense that whatever tension you are already used to, that same tension, approximately, is retained for each string in the new tuning regime.
You need to experiment a bit, by going up in gauge as a particular string goes down in pitch. There are individual strings available to facilitate this.
One set of strings for two rather different tunings isn't going to be anything but a compromise. As Todd says, a second guitar is a far beter option if you want to do that. Apart, faffing about re-tuning, unless done smartly, always sounds so amateur, and wastes a lot of time. Then after, the same faffing to re-tune..!
40 odd years ago, I made the same sort of decision, and from then use a hybrid set on several guitars - unfortunately not avaiable as a complete set of strings off the shelf - but in standard tuning, it works for me. Others hate it, and it means I'm so used to the feel, it's not pleasant playing others' guitars, but there we have it. We can set up our own guitar action, (or have it done for us), so why not have a bespoke set of strings to boot?