My daughter experienced a very similar situation, although her renin levels were only relatively high . I found an article in the CAH support forum webpage, written by a ped endo. I am pasting the link to the article below as well as the relevant info. Hope this helps!

http://www.cahsupportforum.com/treatmentofcah.html

"Usually there are no symptoms from being on too little fludrocortisone. However stopping fludrocortisone in salt wasting CAH results in a marked drop in sodium and an increase in potassium. This is dangerous and potentially fatal. If the fludrocortisone dose is a little low the resulting increase in renin and angiotensin 2 can stimulate the adrenal to increase androgen production (even when the glucocorticoid dose is appropriate or excessive).

Too much fludrocortisone can lead to an increase in extra cellular fluid and this can cause high blood pressure. For optimum control of salt wasting and to avoid the complications of under or overdosing, the dose of fludrocortisone needs to be monitored regularly by checking renin levels.

Some patients seem to need twice daily dosing. This can be established from renin levels that if elevated on one day dosing can fall in to the normal range on twice daily dosing" ....

..."The typical dose of fludrocortisone is 0.05-0.2 mg daily. Babies and infants need higher doses of mineralocorticoid. The dose is usually 0.1-0.2 mg daily. During later childhood less is needed, usually 0.05-0.1 mg daily. A relatively common mistake is failing to reduce the fludrocortisone as children become more sensitive to it. Determination of this dose chanqe is based on suppressed renin levels and elevated blood pressure."