Hi!
First, congratulations! The decision to take a first step into lith printing is nothing you will regret. Rather the opposite. It is addictive. Really.
Well, the instruction for lith (paper) developer generally means a mix between concentrate and water to add. Arista most likely suggest this as well so a mix 1:24 means 1 part concentrate, which consist of 50% of A and B respectively. I.e 0.5A+0.5B+24 water.
However, this is just a guide. Lith is an extremely flexible and variable method and any mix should be taken as a recommendation only. You don´t have to be accurate at all really but still remember what you do and what mix you use for a specific result. You can also vary the A/B mix, i.e 0.3A+0.7B+24water...or else. This make sense as well.
There are many parameters to adjust to achieve different result and mix is only one of many many. Changing one parameter at time will help to understand the effect of them. Initially you could adjust exposure time, snatch point and dilution/mix. Further on you can try higher temperature of the developer, adding restrainer, adding other additives. Once through this cycle, with the very same paper in use, you can switch to another paper and learn the differences between brands and emulsions.
I suggest to start with 1:24 as per recommendation and review the result. Bare in mind the two Golden Rules
http://unblinkingeye.com/Articles/Lith/lith.html and enjoy your discoveries. Take notes! Exposure time, developing time, developer temperature, enlarger height, lens in use, condensor (if using different), aperture, paper, developer mix etc. This will be of GREAT benefit if later aiming at reproducing a lovely print.
1. Make a few prints with same exposure and change the snatch point, i.e sooner/later and learn the effect from that.
2. Once you get the effect of snatch, you can try different exposures. Add one step or reduce one step as a start. With the same snatch. Learn the result.
3. Change dilution, i.e the mix. Try double to 1:48. Might be a good idea not to use already exhausted developer since developing times might be long then. Start over with fresh developer with higher dilution.
I am curious Arista Lith Developer. I haven´t tried it but due to the advantageous price (dollar vs SEK) I´d like to get it a try. ...if US admit delivery to Sweden of course.
Good luck!
/P