You might want to consider doing away with the booster and go for a proper seat. I never understand why in this information age parents are keen to fit their children into these backless boosters as soon as possible. All research shows they are not the safer option. My six year old, 130cm son still fits into his stage 2/3 car seat and is likely to fit in if for at least two/three more years. The backless booster seats offer very little protection or support for their little frames.
From childseatsafety.co.uk and echoed on many other sites.....
"High backed boosters offer upper body protection for the neck, spine and head and help position the seat belt correctly across the child’s chests and hips. In a collision, especially a side impact, a booster cushion wouldn’t offer this protection and therefore it is recommended that a child uses this type of seat until they reach at least the minimum legal requirement of 135cm in height".
In Scandinavia the law dictates they are rear facing until 7 I think!
Any reputable shop will let you try out a few seats in your car, and if they won't, why would you want to buy something so important from them anyway.