In Scheme language, how to assign a variable count

I want to read 10 case files in Ansys Fluent and for each case file there are 10 data files to be read. Ansys Fluent uses Scheme programming language. I have to managed to get some answers to individual problems in the code here (Evaluating a floating point variable in Scheme language) and here (How to increase counter in a do-loop within Scheme language?), but when collecting the individual answers I realized that I need a new code for the counter which is used to read the data files through do-loop. Here is the code with solutions from from other question included:

(do ((i 10 (+ i 1))
     (j 5  (+ j 1)))
    ((>= i 20) 'my-return-value)
  (ti-menu-load-string 
   (format #f "/file/read-case \"C:/DataProcessing/Case~a-time~a-sec/test/Case~a-time~a-sec.cas\"" i j i j))
  (do ((datafilenum 5.100 (+ datafilenum 0.100)))
      ((>= datafilenum 6.000))
    (ti-menu-load-string (format #f "/file/read-data \"C:/DataProcessing/Case~a-time~a-sec/test/Case~a-time~a-sec-~.3f.dat\"" i j i j datafilenum))
    (ti-menu-load-string (format #f "/plot plot n \"C:/DataProcessing/Case~a-time~a-sec/test/water-vof/column-water-vof-at-~.3fs.txt\" y n n water vof y 0 1 0 16 ()" i j datafilenum))))

What I'm trying to achieve here is: read the case file

Case10-time5-sec.cas

then it reads the 10 data files and plot the answer

Case10-time5-sec-5.100.dat
Case10-time5-sec-5.200.dat
...
Case10-time5-sec-6.000.dat

Next loop:

Case11-time6-sec.cas

read the 10 data files and plot answer

Case11-time6-sec-6.100.dat
Case11-time6-sec-6.200.dat
...
Case11-time6-sec-7.000.dat

Next loop...

So, how to change datafilenum starting with 5.100 in this code to 6.100, 7.100, 7.100 etc. when j changes value in the upper loop, something like j.100 and append this value to the exported text file column-water-vof-at-~.3fs.txt. And, of course change, 6.000 to 7.000, 8.000..., something like j+1.000? This got me very confused as I have used trial and error to achieve it!

How to get the number. If j is 6 and you want 6.1 you add 1/10 to it using standard math operations.

(define j 6)
(+ j 1/10)
; ==> 61/10 (aka 6.1 exact)

The function format is not standard and thus there are many competing implementations. In SRFI-48 Intermediate Format Strings you can do this to get 61/10 to be displayed as 6.100:

(format #f "~0,3F" (+ j 1/10)) 
; ==> "6.100"

So putting it all together:

(do ((i 10 (+ i 1))
     (j 5  (+ j 1)))
    ((>= i 20))
  (ti-menu-load-string 
   (format #f "/file/read-case \"C:/DataProcessing/Case~a-time~a-sec/test/Case~a-time~a-sec.cas\"" i j i j))
  (do ((datafilenum (+ j 1/10) (+ datafilenum 1/10)))
      ((>= datafilenum (+ j 1)))
    (ti-menu-load-string (format #f "/file/read-data \"C:/DataProcessing/Case~a-time~a-sec/test/Case~a-time~a-sec-~0,3F.dat\"" i j i j datafilenum))
    (ti-menu-load-string (format #f "/plot plot n \"C:/DataProcessing/Case~a-time~a-sec/test/water-vof/column-water-vof-at-~0,3Fs.txt\" y n n water vof y 0 1 0 16 ()" i j datafilenum))))

If this isn't working you need to edit in which implementation you are using. eg. Racket has format which is different, but it also supports SRFI-48 so I tested this with (require srfi/48). I prefer to use a SRFI rather than the implementations version since porting to a different implementation or revision of RNRS later will be easier.