Newbie question in dilution ratios.

Welcome to Photrio.
You may find that the recommended volumes are visible on the bottom of the tank.
Your arithmetic is correct, but it is more precise than it need be for that chemical and that dilution.
If the recommended volumes are 290 ml and 580 ml of working strength Rodinal developer, it will be fine to use 300 ml and 600 ml instead.
And for something like Rodinal used with a single tank, there is no functional difference between 1 + 25 and 1 + 24, so you may as well just use 12 ml of Rodinal topped up with water to 300 ml for one roll, and 24 ml of Rodinal topped up with water to 600 ml for two rolls.
There are mixtures that require more precision, and as you gain experience you will recognize them.
As far as disposal is concerned, most of the chemicals used in black and white darkrooms are fairly benign. There may be specific local regulations that need to be adhered to, and how your waste water is dealt with (municipal sewer vs. septic field) matters, but used working strength Rodinal is something I wouldn't hesitate to discard down my drain.
Stop bath is quite benign - in many cases just dilute vinegar. Fixer is something that is designed to be re-used, and should really be disposed of properly, as it contains silver which is an anti-bacterial agent.

AsdaFan said:

I'm just starting with film and I've ordered a b&w developing kit which appears to have everything in it apart from 1L beakers. It's not yet been delivered.

My first question is in regards to dilution ratios as for example an Agfa APX 400 says 1+25 for ISO 400 with Rodinal. I'm not sure how much solution a Paterson tank can hold but found a site saying for a single roll the total solution is 290ml and for two it's 580ml, so am I right in saying:

290 / 26 = 11.15ml of Rodinal
290-11.15 = 278.85ml water
Total solution = 290ml

How accurate do you have to be with measurements? In regards to the above, could I round to 12ml of Rodinal and 280ml of water?

Lastly, where do you folks dispose of used chemicals? Is it safe to pour down the sink and run the tap for a minute?

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Yes to all your questions but, there may be local restrictions about chemical disposal.Check with local authorities.

You can always mix a slightly larger quantity than you need, just don't overfill your tank. So for 1+25, you can do 14+351 for a total of 365ml. Just use what fills the tank, dispose of the rest along with the used developer. Rodinal is nasty stuff according to their data sheet, so I would see if your community has some sort of hazardous waster disposal service.

Pieter12 said:

Rodinal is nasty stuff according to their data sheet, .

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I believe that the concern with Rodinal is that the active ingredients are basically dissolved in a LYE solution, which would be quite corrosive as a concentrate, (somthing like LYE Toilet cleaner) but not too bad once mixed. but yes, check you local rules.

for 290 ml, I would just mix 300ml and their should be room for that in the tank.

Be careful about "filling" the tank.
It is important that there be some air space above the liquid in the tank. When you agitate by inverting the tank, the liquid should tumble through the film. If there is too much liquid in the tank, the mixing that happens when you agitate can be impeded.

Thanks for all your answers!

Make it easy on yourself. Go for 300ml for one 35mm roll, 600ml for two, 500ml for one 120 roll.

There's a very practical volume mixer on the massive dev chart: https://www.digitaltruth.com/devchart.php?doc=volmix

^^^^^ Lots of good advice above. ^^^^^
Whichever method you use for mixing your Rodinal, BE CONSISTENT in your method...