Teaching alone time

Hi everyone. I have a ~17/18 weeks old Staffy cross adopted from a shelter. She has been with us for over a month. We have been slowly trying to build up alone time, so she gets used to it before we go back to work. We usually go for a long walk in the morning, then I let her settle down a bit, and before I leave I give her a kong and say 'see you' without making any fuss. We record her while we are gone. I leave the kong in the leaving room, where she has one of her bed and where she eats. She sleeps in the bedroom with us and has another bed in there, and that is a more quiet and enclosed room so we recently put the bed in a crate and created a 'safe contained spot' for her - but we never crated her and she has access to the whole house and backyard because so far she hasn't done any damage and she is not so much of a chewer (she sometimes grabs shoes but just brings them in her bed..). I have got up to 1 hour and 15 minutes (plus one accidental longer time that we had to go to the hospital and we left her for 2 hours and a half, but she was outside and did not realise we were gone.). For the first ~30 minutes, she is busy with the kong and doesn't really care that we are gone. When she finishes the kong, I noticed that she looks around a bit, and goes in her crate for sometime, and comes back in the leaving room. She owls every now and then - but not constantly - at very occasionally bark. I just wonder if this is okay? She doesn't seem super stressed, but she is howling sometimes so I assume she feels a bit upset? I am thinking to keep to 1 hour - 1 hour and 30 minutes for some time and see how she goes. Would this be ok to is it too much? Also, what do you think about giving a kong every time we leave? I was trying to make the time alone pleasant, but I just wonder whether it is okay to give it every time. The kong is filled with her lunch so I shouldn't be over feeding her. During the day while we work she is usually by her self in another room or outside. we blocked the access to the 'office' room with baby gates because there are many cables and also we keep our cat's food bowl in there. But of course she knows we are there. Then at lunch time we spend some time together. I find it hard to know whether I am progressing in the right way so suggestions would be really appreciated. Thanks so much in advance.