I have uploaded the excel file we used today. Maybe you can decrypt my message on sheet "decrypt with statistics"
By the way. When using a shift only for one letter and then change the shift for the next letter you will get 100% safe encryption. drawback: your recipient has to know your method too and this is why it gets unpractically. Maybe you can use this method for really important messages. See
https://en.wikipedia.org/wiki/One-time_pad