Hi Mark, no right or wrong here, its up to you partly and depends also if yu find soil or gravel under the concrete, if the latter I would put at least 6 inches compost. I find better results from all or at least three quarters compost to soil, the latter is nutrient-weak and often poor quality if bought.
If good soil, 4 inches compost is enough but more won’t hurt. A tunnel can grow a lot of veg and will repay you for enriching the soil.